Phagocytosis of lymphocytes undergoing apoptotic death is important for the clearance of dying cells before they release potent inflammatory mediators into the vasculature or tissues. Apoptosis occurs during normal lymphocyte development and viral infections as well as following lymphocyte exposure to endogenous corticosteroids released during stress, malnutrition, and trauma. The investigator has recently discovered that apoptotic cells are phagocytosed by high endothelial venule (HEV) cells within lymph nodes of corticosterone-treated, but not control mice. Since HEV cells are specialized endothelial cells that regulate lymphocyte migration into peripheral lymphoid tissues, they may provide an important checkpoint for clearance of apoptotic lymphocytes within the vasculature, in addition to limiting entrance of nonfunctional lymphocytes into tissue. HEV cell phagocytosis of apoptotic T cells may also be deleterious during the pathogenesis of AIDS. HEV cell phagocytosis of cells infected with human immunodeficiency virus (HIV) may be a route by which HEV cells are infected with HIV, becoming a reservoir of HIV for viral spread during lymphocyte migration across HEV cells. To examine the molecular mechanisms of HEV cell phagocytosis, the principal investigator has established an in vitro model with cell lines derived from murine lymph node HEV cells, and shown that these HEV cell lines bind and promote the migration of viable lymphocytes in an in vitro migration assay and will specifically recognize and phagocytose apoptotic leukocytes. These unique cell lines will be used to 1) define the stage of apoptosis at which the mHEV cells recognize apoptotic leukocytes, 2) identify the receptors involved in mHEV cell recognition of apoptotic cells by using competitive inhibitors for phagocytic receptors in combination with a novel flow cytometric assay which has been developed to differentiate between lymphocyte surface binding and lymphocyte internalization, 3) determine whether phagocytosis via these receptors is modulated by cytokines, and 4) determine whether mHEV cell phagocytosis of apoptotic cells alters HEV cell promotion of lymphocyte migration. Primary cultures of human tonsil HEV cells will be used to determine whether HIV survives HEV cell phagocytosis of apoptotic HIV-infected lymphocytes.